I would guess the frame was made about 1950. Not too much earlier on account of the headset; earlier frames typically had what looks like an integrated headset (the bearing cups fit into a recess in the head lugs). The lugs appear to be what's called "bilaminate" construction, which I won't explain because
this page does it better than I could.
I would certainly look at all the photos of Claud Butler frames, to see if you can find one like this; but I suspect it's another builder, such as Mercian. Compare
this bike, for example; it's made with Nervex lugs plus bilaminate extensions similar to what's on OP's frame.
Oh man, Claud Butler was never even on my radar before, but now I really want one. I'm assuming they're hard to find and when you do find one, it's expensive.
Just out of curiosity, what makes you think this bike isn't a Butler? It seems to tick all the right boxes from what I can tell. Unless, did Claud Butler sell lug sets to other builders?